Fiber-optic angular rate sensors, i.e. gyroscopes, are being implemented to replace mechanical angular rate sensors used for guidance, control and navigation purposes. Fiber-optic devices turn on instantly, have long shelf life and are virtually maintenance free. As "strap-down devices" they do not require expensive gimballed mounting systems and are not encumbered by low rotational rate lock-in that causes other optical angular rate sensors to provide inaccurate outputs at low angular rates.
Fiber-optic angular rate sensors exploit George Sagnac's concept of exciting an interferometer from an external optical source and directly measuring the phase shift of two counterpropagating light beams after they traverse a closed path. In the case of a fiber-optic device the path is a glass fiber instead of Sagnac's original free space, and the effective sensitivity of the device is enhanced by using multiple turns of fiber in a coil arrangement.
Basic fiber-optic angular rate sensors use a beam from a single optical source and divide the beam into two beams. The two beams are directed in opposite directions (counterpropagated) through a multi-turn fiber-optic coil. Rotation of the coil produces Sagnac phase shifts in each beam that are equal in magnitude but opposite in sign. The phase difference between the two optical beams is linearly proportional to the rotational rate of the coil. Most configurations use a phase modulator to increase the sensitivity of the device for small angle rotations and to reduce noise by measuring the phase at the modulation frequency.
The principals of the above described technology are described in an article entitled Fiber-Optic Gyroscopes by B. Y. Kim and H. J. Shaw published in the March, 1986 issue of "IEEE Spectrum", pages 54-60, the same being incorporated herein by reference.
Fiber-optic gyroscopes of the type described above use acousto-optic modulators in each arm of the interferometer. Rotational rate of the sensor is determined by the frequency difference between each modulator. Since each modulator operates in a frequency range near 100 megahertz, frequency measurements to within several hertz are difficult and costly to implement.
The present invention provides the frequency difference and the sign of the frequency difference by using electronic mixers. Since the maximum frequency difference is in the order of several megahertz, standard electronic counters can be used to accurately determine the decreased frequency.